muscle fibres Flashcards
What is the basic structural unit of muscle tissue?
The muscle fiber (or muscle cell), which is a long, multinucleated cell containing many myofibrils
What are myofibrils?
Rod-like structures within muscle fibers, composed of repeating units called sarcomeres, which are responsible for muscle contraction
What is the sarcolemma?
The plasma membrane of a muscle fiber, which surrounds each muscle cell and plays a role in conducting electrical impulses.
What is the role of the endomysium in muscle organization?
A thin layer of connective tissue that surrounds individual muscle fibers, providing structural support and isolating each fiber
What is the perimysium?
A connective tissue layer that groups muscle fibers into bundles called fascicles, allowing coordinated contraction of muscle fibers within each bundle
What is the epimysium?
The outermost layer of connective tissue that surrounds the entire muscle, protecting it and helping transmit force to tendons
What is a sarcomere?
The smallest contractile unit of a muscle fiber, made up of myofilaments and extending from one Z-disc to the next
What are the main myofilaments in a sarcomere?
Thin filaments, primarily made of actin, and thick filaments, primarily made of myosin, which interact to produce contraction
What is the Z-disc (or Z-line)?
A protein structure that anchors the thin filaments and defines the boundaries of each sarcomere.
What is the I-band in a sarcomere?
The lighter region around the Z-disc where only thin (actin) filaments are present, which shortens during contraction
What is the A-band in a sarcomere?
The darker central region of the sarcomere where thick (myosin) filaments are located; its length remains constant during contraction
What is the H-zone in a sarcomere?
The central part of the A-band where there are only thick (myosin) filaments, which decreases in size during contraction.
What is the M-line?
The line in the center of the H-zone where myosin filaments are linked, helping stabilize the thick filaments within the sarcomere
How does the sarcomere facilitate muscle contraction?
Myosin heads on thick filaments bind to actin on thin filaments and pull them toward the center of the sarcomere, shortening the muscle fiber
What is the role of titin in sarcomere structure?
A large elastic protein that connects the Z-disc to the M-line, providing elasticity and helping align myosin filaments.
How are sarcomeres arranged within a muscle fiber?
Sarcomeres are aligned end-to-end along the length of each myofibril, allowing for coordinated contraction across the entire muscle fiber.
How do fascicles contribute to muscle structure?
Fascicles are bundles of muscle fibers encased by perimysium, organizing fibers within the muscle and enabling forceful contractions.
What is the role of actin in muscle contraction?
Actin forms the thin filaments in the sarcomere, providing binding sites for myosin heads during contraction, allowing cross-bridge formation and muscle shortening
What is the role of myosin in muscle contraction?
Myosin forms the thick filaments in the sarcomere; its heads bind to actin filaments, pulling them inward through power strokes, which generates the force needed for contraction.
How are actin and myosin filaments arranged in the sarcomere?
Actin (thin) filaments are anchored to the Z-discs, while myosin (thick) filaments are anchored at the M-line in the center of the sarcomere. The overlapping of these filaments enables the sliding mechanism of contraction.
What is the sliding filament theory?
A model describing muscle contraction where actin and myosin filaments slide past each other, shortening the sarcomere without changing the length of individual filaments.
What is a cross-bridge in muscle contraction?
A connection formed when the myosin head binds to actin on the thin filament, initiating the power stroke that generates contraction.
How does ATP facilitate muscle contraction?
ATP binds to myosin, allowing the myosin head to detach from actin after a power stroke. Hydrolysis of ATP then “re-cocks” the myosin head, readying it for the next cross-bridge cycle
What is the power stroke in muscle contraction?
The action of the myosin head pulling actin filaments toward the M-line after binding to actin, causing sarcomere shortening and muscle contraction
How is calcium involved in muscle contraction?
Calcium ions bind to troponin on the actin filament, causing a conformational change that moves tropomyosin away from myosin-binding sites on actin, enabling cross-bridge formation.
What is the role of tropomyosin in muscle contraction?
Tropomyosin is a regulatory protein that blocks myosin-binding sites on actin when the muscle is relaxed, preventing contraction
How does troponin regulate actin-myosin interactions?
Troponin binds calcium ions during muscle activation, causing tropomyosin to shift, which exposes myosin-binding sites on actin and allows contraction to occur.
What happens to the sarcomere during contraction?
The sarcomere shortens as actin and myosin filaments slide past each other; the I-band and H-zone decrease, while the A-band remains constant
How does sarcomere shortening lead to muscle fiber contraction?
As each sarcomere shortens, it pulls adjacent sarcomeres closer together, resulting in the overall shortening of the muscle fiber and generation of contractile force
How do multiple sarcomeres contribute to whole-muscle contraction?
Sarcomeres are organized in series along myofibrils; simultaneous shortening of all sarcomeres in a myofibril leads to coordinated muscle fiber and whole-muscle contraction
What is the role of the Z-disc in sarcomere organization?
The Z-disc anchors actin filaments and transmits force generated by the sliding of filaments to adjacent sarcomeres, maintaining structural integrity during contraction.
What are actin-binding proteins (ABPs)?
Proteins that interact with actin filaments to regulate their organization, stability, and dynamics, which are essential for muscle structure and contraction
What role does tropomyosin play as an actin-binding protein in muscle cells?
Tropomyosin wraps around actin filaments and blocks myosin-binding sites, preventing contraction in a relaxed muscle state
How does troponin function as an actin-binding protein?
Troponin binds calcium ions and, upon activation, shifts tropomyosin away from myosin-binding sites on actin, allowing contraction to occur.
What is the role of nebulin in actin organization?
stabilizes actin filaments, thereby regulating filament length
How does titin interact with actin and myosin?
act as springs to keep myosin filaments centered in the sarcomere.
What is the function of α-actinin in muscle cells?
α-Actinin is an actin-binding protein located at the Z-disc, where it cross-links actin filaments from adjacent sarcomeres, maintaining alignment and structural stability.
How do capping proteins regulate actin filaments in muscle?
bind to the ends of actin preventing further polymerisation or depolymerisation
What is the role of tropomodulin in muscle cells?
Tropomodulin caps the pointed (minus) end of actin filaments, preventing depolymerization and thus stabilizing the actin filament length within sarcomeres
How does the actin-binding protein cofilin affect actin dynamics?
promotes depolymerisation and turn over
What role does myosin light chain kinase (MLCK) play in muscle contraction?
MLCK phosphorylates the myosin light chains, increasing myosin’s interaction with actin and enhancing contraction, particularly in smooth muscle.
How does calmodulin interact with actin-binding proteins?
Calmodulin is a calcium-binding protein that activates enzymes, including MLCK, to regulate contraction in response to calcium levels
What is the role of dystrophin in muscle cells?
Dystrophin links actin filaments to the muscle cell membrane, providing structural stability and protecting the membrane from damage during contraction.
How does the protein vinculin contribute to muscle cell integrity?
connects actin filaments to integrins at cell adhesion sites
How do actin-binding proteins contribute to sarcomere stability?
stabilize actin filaments at the Z-disc and within sarcomeres, maintaining consistent filament length and organization
